The present invention relates generally to granulating methods and apparatus for forming powder materials into granules and, more particularly, to a novel method and apparatus by which substantially spherical granules of a substantially uniform size may be formed from a powder material substantially without applying an agglomerating liquid or binder.
In various and diverse technologies, raw material processing conventionally requires the initial processing of raw material in powder form into larger agglomerated granules which may be more easily handled. Preferably, such agglomerated granules should be substantially spherical in shape and as uniform as possible in composition and density for ease of flowability and handling of the granules and to enhance the quality of end products formed therefrom. On the other hand, it is desirable that such granules be adapted for relatively easy disintegration back into powder form when the granules are to be subjected to further processing, e.g., mixing, kneading, or pressing operations or the like.
Various forms of granulating apparatus have been developed over past years. In granulating apparatus of the type commonly referred to as pan granulators or rotating drum granulators, a charge of powder material is continuously subjected to a rolling and cascading motion within the granulator vessel while a liquid binder is introduced to adhere the powder material. As the rolling and cascading motion of the powder material continues, the material is formed into generally spherical granules. However, disadvantageously, the granules produced in this type of granulating apparatus vary in granule diameter over a relatively wide range, e.g., between granules of several millimeters and granules of several centimeters. Accordingly, it is difficult in the use of such apparatus to obtain granules of smaller diameters, e.g., below several millimeters, and also to obtain uniformity in the size of granules produced. Further, problems may be encountered in the operation of this type of granulating apparatus in controlling the moisture content of the granules, which contributes to the wide range in granule size.
In spray drying granulators, commonly referred to as spray dryers, a slurry composed of powder raw material and a binder or other agglomerating liquid is continuously sprayed in droplet or globular form into a vessel while heated drying air is directed through the vessel to drive off the agglomerating liquid, thereby transforming the globules into essentially dry granules of the powder material.
While this apparatus and method produces generally uniform spherical granules with acceptable flowability characteristics, it is difficult to produce relatively dense granules due to the heat-induced evaporation of the agglomerating liquid which tends to cause outward migration of the finer particles of the powder material toward the granule surface. Accordingly, the granules produced are often hollow and of relatively low density for their volume. Further, problems may be encountered in achieving consistent homogeneity and powder material content in the preparation of the slurry for this type of apparatus. The overall size of spray dryers is relatively large and therefore not generally suitable for frequent material changes because of the necessity and difficulty of cleaning the entire apparatus system.
Extruding machinery is also available for forming moistened powder material into generally cylindrical pellet-type form by the forced extrusion of the material through a die or screen. Disadvantageously, it is difficult to produce pelletized granules by this operation which are less than about one millimeter in diameter. Moreover, the cylindrical shape of the pellets significantly restricts their flowability and, as a result, such pellets are often further processed in a spheronizing vessel wherein the pellets are repetitively beaten against the vessel walls by a rotating plate to deform and plasticize the pellets into spherical form.
Fluidized bed granulating machines have been developed which essentially combine the functions of an extruder, a spheronizer and a dryer for granule formation. In such apparatus, powder raw material is fed into a cylindrical vessel having a stationary screen or plate with openings therethrough, the powder material being treated with an agglomerating liquid spray on the charge side of the screen or plate while heated drying air is forced through the screen or plate openings from the opposite side thereof to form the powder into dry granules. Disadvantageously, such granulating apparatus produce a widely varying range of granule shapes and sizes which inhibit good flowability of the granules.
In another form of agitative-type granulating apparatus, a charge of powder material is processed in batch form in a vessel having driven rotors for circulating and mixing the powder charge material, with an agglomerating liquid spray being provided for forming the powder material into granules as circulation progresses. Hereagain, the granules produced by such apparatus typically range widely in granule size and, further, the apparatus is poorly capable of accurate repeatable control of granule size.
Other machines are available for compacting powder material mechanically between a pair of rollers to form relatively dense granules or brickettes. However, the granules or brickettes produced by these apparatus also range widely in cross-sectional dimension from several millimeters to several centimeters and are generally of a non-spherical shape.